The present invention relates to a device for processing workpieces wherein a lower tool, which receives the workpiece, can be moved up against an upper tool by a lifting frame such that the workpiece can be processed in this position. More particularly it relates to such a device having an improved lifting frame which permits the upper and lower tools to be changed easily. The present invention is particularly useful for devices which do multispot-welding.
Congeneric devices are usually used as multispot-welding equipment. The presently used devices have a frame which usually has four perpendicular columns. The lower section of the frame accommodates a lifting device between the columns. The lifting table of this lifting device carries the lower tools. When the lifting table is lowered, the workpieces can be placed onto the lifting table and among the lower tools. When the lifting table is lifted, the workpiece is presented to the upper tools for processing.
Typically, the upper tools hang on a head plate which rests on the columns, while the lower tools are connected to the upper tools (for example, by assembly columns) such that when the tools are to be exchanged, the complete tool set (both upper and lower tools) is lifted out of the frame by means of a crane. The tool changes in these devices are very time-consuming, since many parts have to be released, fastened again and also set up for a different workpiece or process. Also, a large amount of head room above the device is required since the columns of the frame extend from the floor up to the head plate and during changing the tool set has to be moved such that its lower edge passes over the top of the columns. Thus, it would be desirable to have a device which overcomes these problems by improving the tool-changing efficiency while maintaining or improving the quality of the work.
The present invention provides a device which is horizontally subdivided into a tool frame and a lifting frame. The tool frame comprises a plurality of perpendicular columns, a head plate connected to the columns and a base plate detachably fastened to the columns. The tool frame can be mounted on the lifting frame in an easily releasable manner. In the mounted state, the connection between the base plate and the columns can be released and a connection can be made between the base plate and the lifting table of the lifting frame. This creates two constructional groups, namely, the lifting frame and the tool frame which can be quickly connected to one another and also separated again and which can optimally fulfill their different tasks.
In particular, the present invention enables a different tool frame to easily be made available for the different workpieces to be processed. It is possible with the present invention to make an external adjustment to the tools or the tool frame which does not impede the production process. Additionally, the present invention provides a quick-changing means which, compared with that used in known tool-changing devices, only requires a fraction of the resetting time, thereby making it possible not only to convert the device upon completion of a production series but also to meet other stated requirements during the course of production, such as continuous or batch production.
Another advantage of the present invention is that the tool frame, upon completion of a series of steps on a workpiece, can be easily used for other tasks, such as for spare-parts production. The tool frame, with both the upper and lower tools, can also be stored for use at a later time or it can be conveyed to another production site where it is mounted onto an identical lifting frame for immediate use. If the tool frame is not to be stored for use at a later time, it can continue to be used for another purpose by converting or exchanging the tools secured on the tool frame. Also, the lifting frame can be fully reutilized for constructing a new installation.
The device according to the present invention uses a system of modular construction wherein the individual components (such as lifting and tool frames) can be set up or used at different locations. When combined, these different components form different, fully functionable devices. Moreover, the lifting frames, since they do not contain any components specific to the tools or workpieces being used, can be used universally without any resetting. It is the tool frames which contain the components and tools that are specific to the different workpieces. The tool frames can be made available in sufficient number for each workpiece such that when a workpiece is finished being produced, the tool frame can be converted without difficulty to one needed to work on a new workpiece. During this conversion process, no parts of the actual tool frame necessarily need to be replaced.
When a tool frame is to be mounted onto the lifting frame, the detachable base plate of the tool frame carrying the lower tools must first be fixed to the tool frame. Then the tool frame, with its columns, is placed onto the lifting frame and fastened thereto. Finally, the base plate is secured to the lifting table and then the connection between the base plate and the tool frame is released. This creates a unit which can move vertically inside the columns, which are now continuous. The unit is readily accessible and in the area wherein the tool frame adjoins the lifting frame requires no additional connecting elements between the columns despite the means of separation.
Preferably, a conveying means or a conveying device is attached to the tool frame and can insert the workpiece into the tool frame and remove it therefrom. The conveying device, except for the conveying arms contacting the part, can be standardized and thus be used in every tool frame. The conveying travel can be universally set via control cams. The drive of the conveying device is conveniently effected using a change-pole geared motor which ensures a high positioning accuracy. Optimum adjustment outside the production line under operating conditions as well as a quick change-over are also possible with this device.
So that a quick change of the tool frames can be undertaken without the aid of additional equipment such as a crane or a fork-lift truck with their considerable space requirement, it is preferable to have another tool frame on one of the two storage devices which directly adjoin the sides of the lifting frame. With such a arrangement, it is possible to carry out a quick change between two tool frames and thus to change to another workpiece without a great loss of time.
Each storage device preferably has a displacing unit for moving a tool frame from the lifting frame to the storage device and vice versa. This changing operation for the tool frame can be accomplished by just one person or it can be automated. The time-savings created by this tool-changing operation considerably increases the productivity of the machine. This becomes quite evident when the device according to the present invention is compared with previously used devices in which time-consuming assembly, setting and transport operations have to be carried out. Setting or resetting in the prior devices takes up so much time that it can only be carried out during prolonged production stoppages.
Preferably, the storage devices, on their upper side, carry a support means such as hardened strips, rollers or balls for ease of displacement and also have a locking mechanism for the stored tool frame. The easy displaceability resulting from the use of the hardened strips, rollers or balls means that only small drive forces are required and thus little energy and construction effort. On the other hand, easy displacement necessitates the use of a locking mechanism, which however, can easily be effected by mechanical or automatic pinning.
Preferably, a crank mechanism is used as the drive means for lifting and lowering the lifting table. The crank mechanism, which preferably performs a movement of 180.degree. during the lifting movement or during the lowering movement, provides favorable force ratios and a more accurate means of setting when compared with the known mechanisms for lifting tables such as air cylinders, swing frames, etc. The crank mechanism is preferably driven by a motor-gearbox unit from which protrude shaft sections having first crank linkages to which are attached first crank pins which accommodate supports connected to the lifting table. Use of second and third crank linkages also enables favorable force ratios to be realized with little additional construction effort. A 90% displacement of the second crank linkage relative to the third crank linkage also prevents unfavorable concentration of force at the dead centers.
To achieve a large swivel angle for the crank linkages, at least two non-driven shaft sections are each interrupted and mounted on one side in such a way that a transmission member can pass though in between unimpeded. For the same reason, some or all of the transmission members and/or the supports acting on the lifting table can be arranged in an offset configuration. In practice, it is better if each of the two driven shaft sections is mounted in two bearings, the crank linkages are located between the bearings, and the driven shaft section is constructed so as to be separable via flanges between the motor-gearbox unit and the adjoining bearing. For maintenance and repair, the motor-gearbox unit can easily be pulled out of the lifting frame because of the flanges.
The motor for the motor-gearbox unit preferably comprises a direct-current motor which is controllable via a camshaft gear mounted on a shaft section. The lifting characteristic of the lifting table can be varied in a simple manner using a direct-current motor. Apart from the intermediate positions thereby possible, different speeds can also be applied.
The present invention also enables the lifting table to be guided on the columns of the lifting frame and to interact with the pressure cylinders which support the weight of the lifting table. Thus, the lifting table drive means can produce a smaller output while still lifting the lifting table. The pressure cylinders are conveniently connected to an accumulator in such a way that the pressure medium passes into the accumulator when the lifting table is being lowered and has an assisting effect on the lifting movement when the lifting table is being lifted. The volume of the accumulator in relation to the volume of the displaced pressure medium can be selected in such a way that only slight pressure differences arise during the lifting movement.
Preferably, hydraulic fluid is used as the pressure medium and a hydraulic unit is provided for building up and maintaining the pressure medium. Also, additional devices can be used to adapt the pressure level to various dead loads. The use of hydraulic fluid for the pressure medium ensures the greatest possible freedom from maintenance, since there is self-lubrication. Compared with air as a pressure medium, larger pressures are possible with hydraulic fluid with the result that the equipment is more compact. Also, improved overall efficiency can be obtained since hydraulic fluid, in contrast to air, operates with lower pressure drops. Preferably, the hydraulic unit is electrically driven and is located at the lifting frame. Due to the electric drive, only one type of energy is required for the lifting frame. Moreover, by locating the hydraulic unit at the lifting frame, the construction of a compact device having all necessary constructional groups is permitted.
Depending upon which tool frame is mounted onto the lifting frame, pressure adaptation due to the different weights of the lower tools carried by the lifting table can be useful to operate the device using the least amount of energy. This pressure adaptation to automatically set the pressure required in each case can be implemented in a variety of ways known to one skilled in the art such as by means of valves, pressure switches or other regulating devices which are manually adjustable or controlled by a certain signal. Other known measures and devices are provided to monitor the hydraulic pressure and in the event of a pressure drop of a certain size, automatically start the hydraulic unit and then switch it off. It is also necessary to start up the hydraulic unit when, after production has ended, the pressure is released and the device is restarted such as on the following day.
To accurately fix the tool frame, the lifting frame is preferably provided with centering bolts protruding upwards for interacting with the columns of the tool frame. An accurate and constant location of the tool frame relative to the lifting frame is always achieved by the centering bolts. The base plate of the tool frame is held on the tool frame in an accurately located position. Not until the tool frame is connected to the lifting frame via the centering bolts, however, is the lifting table connected to the base plate thereby ensuring that all the parts are located very accurately.
Means are provided for securing the base plate to the columns of the tool frame which ensure that the tool frame, when it is to be pushed by the storage devices over the lifting frame, can at the same time slide past the centering bolts protruding up from the lifting frame. Additionally, the connection between the base plate and the tool frame can be released and secured again manually without a great expenditure of force.
Preferably, the pins and sleeves used in one embodiment of the securing means also form a centering means when the lifting table is in the upper position. The centering occurs during every lifting movement, namely when the lifting table is moved into the upper position. It is precisely in this position that the centering means is particularly important, since a very accurate location of the toolframe base plate which carries the upper tools, is thereby achieved. This centering means ensures that during every lifting movement the location of the lower tools with respect to the upper tools is accurate to at least 1/10 mm. If the tools are, for example, welding tools, good spot-welding quality and long endurance of the electrodes is established as a result of the constant position of the welding units. The welding quality is still achieved by the present invention even though there is a lower standard for component parts. It is inter alia also the precise matching which occurs in the present invention that enables the optimum adjustment to occur outside production under strict operating conditions.